Method of liquefying a hydrocarbon-rich flow

ABSTRACT

A hydrocarbon-rich flow, such as a natural gas flow, is liquefied by means of an open expander process, where the peak coldness required for the liquefaction and the supercooling of the hydrocarbon-rich flow takes place in that the hydrocarbon-rich flow to be liquefied is liquefied and supercooled in one or more heat exchangers, such as plate heat exchangers, against an expanded partial flow of the liquefied and supercooled hydrocarbon-rich flow. The peak coldness for the liquefaction and supercooling of the hydrocarbon-rich flow is generated by the expansion of at least two partial flows of the at least partially liquefied hydrocarbon-rich flow, the partial flows being evaporated at different pressures.

The present application claims priority under 35 U.S.C. § 119 to German Patent Application No. 10 2004 036 708.6 filed Jul. 29, 2004, the entire disclosure of-which is herein expressly incorporated by reference.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to a method of liquefying a hydrocarbon-rich flow, particularly a natural gas flow, by means of an open expander process, where the provision of the peak coldness required for the liquefaction and the supercooling of the hydrocarbon-rich flow take place in that the hydrocarbon-rich flow to be liquefied is liquefied and supercooled in one or more heat exchangers, preferably plate heat exchangers, against an expanded partial flow of the liquefied and supercooled hydrocarbon-rich flow.

The term “open expander process” applies to liquefying methods by which a partial flow of the hydrocarbon-rich flow to be liquefied, before the actual liquefaction, is branched off, cooled, expanded in a refrigerating manner and is subsequently warmed up and evaporated against the partial flow of the hydrocarbon-rich flow to be liquefied. The evaporated partial flow is then no longer admixed to the hydrocarbon-rich flow to be liquefied but, as a rule, is fed into a so-called low-pressure network.

Methods of the above-mentioned type for liquefying a hydrocarbon-rich flow are implemented comparatively rarely because they are meaningfully used only when a corresponding low-pressure network is available.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary method for liquefying a hydrocarbon-rich flow in accordance with the present invention;

FIG. 2 illustrates an exemplary method for liquefying a hydrocarbon-rich flow where the liquefied and supercooled natural gas flow withdrawn from the heat exchanger is divided into a total of three partial flows in accordance with the present invention;

FIG. 3 illustrates an additional exemplary method for liquefying a hydrocarbon-rich flow; and

FIG. 4 illustrates an exemplary method for liquefying a hydrocarbon-rich flow in which a separation of a nitrogen-rich gas fraction takes place in a second separator.

DETAILED DESCRIPTION OF THE DRAWINGS

A method of the above-mentioned type, which is part of the state of the art, for liquefying a hydrocarbon-rich flow will be explained in detail in the following by means of the embodiment illustrated in the FIG. 1.

By way of a pipe 1, a natural gas flow originating from a natural gas network is fed to a single-stage or multi-stage compressor C1 and is compressed in the latter to a pressure of between 10 and 50 bar. If the natural gas flow is already present at a sufficiently high pressure, this compression will not be necessary. Subsequently, the water is removed from the compressed natural gas flow in a absorptively operating separation unit T. The compressed natural gas flow, from which undesirable constituents were removed, is then withdrawn from the separation unit T by way of the pipe 2 and is divided into two partial flows.

By way of the pipe 3, the first partial flow is fed to the heat exchanger E; is cooled in the latter against process flows to be warmed up, which will be discussed below; and is withdrawn from the-heat exchanger at a suitable temperature by way of the pipe 4. A refrigerating expansion to a pressure of between 2 and 10 bar takes place in the expansion turbine X. The expanded partial natural gas flow is now guided by way of the pipe 5 through the heat exchanger E and in the process is evaporated against the partial exhaust gas flows, which are to be cooled, in the pipes 3 and 7. It thus supplies the coldness required for the cooling of the second partial exhaust gas flow to be liquefied, which is fed to the heat exchanger E by way of the pipe 7. The heat exchanger E can also be divided into several parallel and/or successively connected heat exchangers, which is not shown in FIG. 1 for reasons of clarity.

Carbon dioxide is removed only in the separation unit T′illustrated in FIG. 1 from the above-mentioned partial natural gas flow to be liquefied. In principle, the functions of the two separation units T and T′ can also be implemented in a single separation unit; the latter would then have to be provided before the division of the natural gas flow into the described partial flows.

The partial natural gas flow cooled in the heat exchanger E is fed to a separator D by way of the pipe 8. From the sump of the separator D, a liquid C₂₊-rich hydrocarbon fraction is withdrawn by way of the pipe 9; is expanded in the expansion valve a; and is guided by way of the pipe 10 also through the heat exchanger E, this liquid fraction being warmed up and possibly evaporated in the heat exchange with process flows to be cooled.

The fractions guided through the heat exchanger E, by way of the pipes 5 and 10 and warmed in the process to ambient temperatures, are fed by way of the pipe 6 to their further use, as, for example, the feeding into a low-pressure network.

By way of the pipe 11, a gaseous methane-rich fraction is withdrawn from the head of the separator D, is liquefied and supercooled in the heat exchanger E, and is subsequently withdrawn from the heat exchanger E by way of the pipe 12. The peak coldness required for the liquefaction and supercooling of the latter fraction is provided in that a partial flow of the natural gas flow to be liquefied is expanded in the expansion valve c and subsequently is guided by way of the pipe 14 in the counterflow to the natural gas flow to be liquefied and supercooled through the plate heat exchanger E. In this case, the natural gas flow fed to the heat exchanger E by way of the pipe 14 is evaporated on the so-called cold end of the heat exchanger E at low pressure.

After passing through the heat exchanger E, the warmed-up natural gas flow is fed, for example, by way of the pipe 15, to a single-stage or multi-stage secondary compressor or recompressor C2, is compressed in the latter to the pressure of the above-mentioned low-pressure network and is withdrawn by way of the pipe 16 from the recompressor C2. As an alternative, this natural gas flow can also undergo the desired pressure increase by means of a Venturi tube.

That partial flow of the liquefied and supercooled natural gas flow which is not required for the above-described providing of the peak coldness is throttled in the expansion valve b and is fed by way of the pipe 13 into a storage tank not shown in the figure.

One problem with the method illustrated in FIG. 1 is that the heat exchanger E or its buildability are subject to limits. In principle, plate heat exchangers are restricted with respect to the maximally permissible temperature difference between the individual passages. Nevertheless, in the case of a plurality of applications, plate heat exchangers are preferred because, in comparison to other heat exchanger constructions, for example, wound heat exchangers, they have clear price advantages.

It is also a disadvantage of the described method of operation that the liquefied natural gas flow branched off for providing the peak coldness has to be secondarily compressed or recompressed, which results in considerable additional expenditures of energy.

It is an object of the present invention to provide a method of liquefying a hydrocarbon-rich flow, particularly a natural gas flow, which eliminates the above-mentioned disadvantages.

For achieving this object, a method of liquefying a hydrocarbon-rich flow is disclosed where the peak coldness for the liquefaction and the supercooling of the hydrocarbon-rich flow is generated by the expansion of at least two partial flows of the at least partially liquefied hydrocarbon-rich flow, the partial flows being evaporated at different pressures.

The method according to the invention, as well as further developments thereof, will be explained in detail in the following by means of the embodiments illustrated in FIGS. 2 to 4.

In the case of the embodiment of the method according to the invention illustrated in FIG. 2, the liquefied and supercooled natural gas flow withdrawn from the heat exchanger E by way of the pipe 12 is divided into a total of three partial flows. Two of these partial flows are fed by way of the pipes 14 and 14′ after a prior expansion in the expansion valves c and c′, in turn, to the heat exchanger E and are warmed up and evaporated in the counterflow to the natural gas flow to be liquefied and supercooled. According to the invention, the evaporation of these partial natural gas flows takes place in the heat exchanger E at different pressure levels.

In principle, three or more partial natural gas flows can also be warmed up and evaporated against the natural gas flow to be liquefied and supercooled; however, such a method would only make sense in the case of large processes designed for large liquefaction performances or capacities.

The above-described solution solves the above-mentioned problem of the maximally permissible temperature difference between the passages of plate heat exchangers. Thus, in comparison to other heat exchanger constructions, simpler and more cost-effective solutions can be selected for the heat exchanger E.

As illustrated in FIG. 2, the two expanded partial natural gas flows can be combined again in the pipes 14, 14′ and be fed to the recompression C2. However, in addition or as an alternative, one of the expanded partial natural gas flows can also be fed to the C₂₊-rich hydrocarbon fraction in the pipe 10 and/or to the expanded and warmed-up partial natural gas flow in the pipe 5; illustrated in the figure by pipes 17 and 18 respectively indicated by a broken line. These alternatives have the advantage that the quantity of the gas flow fed to the recompressor C2 is reduced, which results in a correspondingly lower energy demand for the recompression C2.

In this case, pressure maintaining valves d, d′ and d″ respectively can be provided in the pipes or pipe sections 14″, 17 and/or 18.

The quantitative proportion between the two partial natural gas flows is selected or adjusted in the pipes 14 and 14′ will depend on a large number of parameters and marginal conditions in the individual case, the selection of which is well within the capabilities of one skilled in the art.

Another embodiment of the method according to the invention for liquefying a hydrocarbon-rich flow is illustrated in FIG. 3.

In this embodiment of the method according to the invention, the natural gas flow to be liquefied and supercooled in the heat exchanger E, which is guided to the heat exchanger E by way of the pipe 11, is withdrawn at a suitable temperature from the heat exchanger E; before it is fed again into the heat exchanger E, a partial flow of the liquefied natural gas flow is separated by way of the pipe 19. This liquefied partial natural gas flow is throttled in the expansion valve d and is subsequently warmed up and evaporated in the heat exchanger E against the natural gas flow to be liquefied, before it is fed by way of the pipes or pipe sections 20 and 10 to the tail gas pipe 6, in which case a pressure maintaining valve e may be provided.

Also in the case of the method illustrated in FIG. 3, in addition or as an alternative, one of the expanded partial natural gas flows can be fed to the C₂₊-rich hydrocarbon fraction in the pipe 10 and/or to the expanded and warmed-up partial natural gas flow in the pipe 5; shown in the figure by pipes 21 and 22 respectively illustrated by a broken line, in which, in addition, pressure maintaining valves e′ and e″ respectively may be provided.

In the embodiment of the method according to the invention of liquefying a hydrocarbon-rich flow illustrated in FIG. 4, a separation of a nitrogen-rich gas fraction takes place in a second separator D′ to which the cooled and at least partially liquefied natural gas flow is fed by way of the pipe 23, which nitrogen-rich gas fraction is withdrawn by way of the pipe 25 at the head of the separator D′.

From the sump of the separator D′, a methane-rich liquid fraction is withdrawn by way of the pipe 24 and is, in turn, fed to the heat exchanger E for the purpose of the supercooling.

The nitrogen-rich gas fraction withdrawn by way of the pipe 25 at the head of the separator D′ is admixed by way of the pipe 27, in which a pressure maintaining valve f may be arranged, to the C₊-rich hydrocarbon fraction from the first separator D.

If desired or required, a partial flow of the methane-rich liquid fraction leaving the separator D′ by way of the pipe 24 can be admixed to the nitrogen-rich gas fraction in the pipe 25 by way of the pipe 26 in which a pressure maintaining valve f is arranged.

Also in the case of the embodiment of the method according to the invention of liquefying a hydrocarbon-rich flow illustrated in FIG. 4, it applies that an additional or alternative admixing of the nitrogen-rich gas fraction to other suitable process flows—analogous to the methods illustrated in FIGS. 2 and 3—can take place.

As an alternative to the method illustrated in FIG. 4, even the liquid fraction from the separator D′ could be expanded in a refrigerating manner and the gas fraction from the separator D′ could be condensed, supercooled and further used by way of the pipe 12.

In comparison to the methods which are part of the prior art, the method according to the invention for liquefying a hydrocarbon-rich flow, particularly a natural gas flow, has the important advantage that conventional plate heat exchangers can be used. As a result, compared with solutions in which other more complicated heat exchanger constructions are used, more cost-effective liquefaction methods and systems respectively can be implemented.

The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof. 

1. Method of liquefying a hydrocarbon-rich flow, particularly a natural gas flow, by means of an open expander process, where the provision of a peak coldness required for liquefaction and supercooling of the hydrocarbon-rich flow take place in that the hydrocarbon-rich flow to be liquefied is liquefied and supercooled in one or more heat exchangers against an expanded partial flow of the liquefied and supercooled hydrocarbon-rich flow, wherein the peak coldness for the liquefaction and supercooling of the hydrocarbon-rich flow is generated by expansion of at least two partial flows of an at least partially liquefied hydrocarbon-rich flow, the partial flows being evaporated at different pressures.
 2. The method of claim 1, wherein only one of the expanded partial flows is evaporated and then compressed.
 3. The method of claim 2, wherein the partial flows of the at least partially liquefied hydrocarbon-rich flow are separated at different pressure levels from the hydrocarbon-rich flow.
 4. The method of claim 2, wherein the partial flows of the at least partially liquefied hydrocarbon-rich flow are separated at different temperature levels from the hydrocarbon-rich flow.
 5. The method of claim 4, wherein the partial flows of the at least partially liquefied hydrocarbon-rich flow are separated at different pressure levels from the hydrocarbon-rich flow.
 6. The method of claim 5, wherein the one or more heat exchangers are plate heat exchangers.
 7. The method of claim 1, wherein the partial flows of the at least partially liquefied hydrocarbon-rich flow are separated at different temperature levels from the hydrocarbon-rich flow.
 8. The method of claim 7, wherein the partial flows of the at least partially liquefied hydrocarbon-rich flow are separated at different pressure levels from the hydrocarbon-rich flow.
 9. The method of claim 1, wherein the partial flows of the at least partially liquefied hydrocarbon-rich flow are separated at different pressure levels from the hydrocarbon-rich flow.
 10. A Method of liquefying a hydrocarbon-rich flow, comprising the acts of: providing a peak coldness required for the liquefaction and supercooling of a hydrocarbon-rich flow; liquefying and supercooling the hydrocarbon-rich flow in one or more heat exchangers against an expanded partial flow of the liquefied and supercooled hydrocarbon-rich flow; generating the peak coldness for the liquefaction and supercooling of the hydrocarbon-rich flow by at least two partial flows of an at least partially liquefied hydrocarbon-rich flow; and evaporating the partial flows at different pressures.
 11. The method of claim 10, wherein only one of the expanded partial flows is evaporated and then compressed.
 12. The method of claim 11, wherein the partial flows of the at least partially liquefied hydrocarbon-rich flow are separated at different temperature levels from the hydrocarbon-rich flow.
 13. The method of claim 12, wherein the partial flows of the at least partially liquefied hydrocarbon-rich flow are separated at different pressure levels from the hydrocarbon-rich flow.
 14. The method of claim 13, wherein the one or more heat exchangers are plate heat exchangers.
 15. The method of claim 11, wherein the partial flows of the at least partially liquefied hydrocarbon-rich flow are separated at different pressure levels from the hydrocarbon-rich flow.
 16. The method of claim 15, wherein the one or more heat exchangers are plate heat exchangers.
 17. The method of claim 10, wherein the partial flows of the at least partially liquefied hydrocarbon-rich flow are separated at different temperature levels from the hydrocarbon-rich flow.
 18. The method of claim 17, wherein the partial flows of the at least partially liquefied hydrocarbon-rich flow are separated at different pressure levels from the hydrocarbon-rich flow.
 19. The method of claim 18, wherein the one or more heat exchangers are plate heat exchangers.
 20. The method of claim 10, wherein the partial flows of the at least partially liquefied hydrocarbon-rich flow are separated at different pressure levels from the hydrocarbon-rich flow.
 21. The method of claim 20, wherein the one or more heat exchangers are plate heat exchangers. 